1. Field of the Invention
The present invention relates to an image forming apparatus for applying a periodic bias voltage to a development agent bearing member which bears and conveys a development agent, and applies the development agent to an electrostatic latent image bearing member in a development region.
2. Related Background Art
It is well known to apply a periodic bias voltage obtained by superposing an AC voltage component such as a sine wave, a rectangular wave, a triangular wave, or the like on a DC voltage component to a development agent bearing member such as a development sleeve, a development roller, and the like (U.S. Pat. Nos. 4,292,387, 4,395,476, and 4,600,295).
The DC voltage component contributes to fog prevention, improvement of the density of a developed image, and optimization of the line width of a line image, while the AC voltage component activates motion of a development agent in the development region, thus contributing to improvement of the density and gradation of a developed image, prevention of a decrease in line width of a line image, and the like.
On the other hand, the following technique is also known. In this technique, spacer rollers are provided to the two end portions of a development agent bearing member so as to maintain a gap between an image bearing member and the development agent bearing member, and the spacer rollers are pressed against the image bearing member (U.S. Pat. Nos. 4,324,199 and 4,373,468).
After repetitive use of the apparatus, when the spacer rollers and/or the pressing portions between the image bearing member and the spacer rollers gradually wear out, the gap between the image bearing member and the development agent bearing member in the development region changes from an initial value.
In this case, since the periodic electric field strength on the development region changes from an initial value, the line width of a developed line image, and the density of a developed solid or whole image undesirably change from initial values.
Also, an image forming apparatus, which can detachably mount a so-called process cartridge which integrally holds an image bearing member and a development device, and also integrally holds a charger and/or a cleaner, if necessary, is known (U.S. Pat. Nos. 4,975,746 and 5,134,441).
The gap between the image bearing member and a development agent bearing member may vary for individual cartridges due to manufacturing errors. In this case, since the periodic electric field strength varies for individual cartridges, the line width and the density of a developed image undesirably vary depending on the cartridges.